Microfiltration is a commonly used for sterilizing liquids and gases. See for example the present inventor's device disclosed in International application no. PCT/CA91/00056, published Mar. 19, 1992 under no. WO 92/04004. In that device, a liquid sterilizing micro-filter is mounted on the outlet of a storage container to dispense sterile saline. A plunger is used to draw saline up from the container and force it through the microfilter, thereby sterilizing and dispensing the saline. A second micro-filter sterilizes the air which is drawn into the device to replace the ejected saline. Other prior art devices which employ hydrophillic membrane filters to sterilize liquids include those shown in applications no. GB 1,000,248; GB 2,106,877; and U.S. Pat. Nos. 4,938,389 and 4,533,068.
Within the microfiltration industry it is known that the integrity of a wetted microfilter membrane and its accompanying seals can be tested by pressurizing the upstream surface of the filter with air. The presence of air bubbles on the downstream side of the filter at relatively low air pressure indicates the presence of a leak either around the filter or through a defect within the membrane itself. The air pressure can be increased to a point where air is able to penetrate through the pores of the membrane. This is called the "bubble-point" test. It can be used to verify the pore size of the filter membrane, however, air-borne micro-organisms may also pass through the filter along with the air.
Once an integrity test has been conducted, the air on the upstream side of the filter remains trapped. This air could be pushed through the filter with liquid at pressures that exceed the "bubble point" pressure, but this may not be desirable for hand held filtering devices that are intended to deliver multiple unit doses of sterile liquid. The trapped air could also be removed from the upstream surface of the filter by inverting the entire device and allowing gravity to temporarily displace the air. This method may be valid but does entail certain technical disadvantages.
There is therefore a need for a mechanism that is capable of introducing and then removing air from the upstream surface of a hydrophillic filter membrane within a manually operated filtration device so that the operator may assure himself that the filter within the device if fully functional, without leaving an air lock on the filter.